1. Field of the Invention
The present invention relates to a method of and an apparatus for detecting a toner-empty state in a developing unit.
2. Description of the Related Art
In an image-forming apparatus such as a copying machine, a printer, a facsimile, etc., a latent image forming apparatus, like an electrophotographic apparatus, has been utilized from a demand for recording on ordinary paper. In such an image-forming apparatus, an electrostatic latent image is formed on a photosensitive drum. Thereafter, the electrostatic latent image on the photosensitive drum is developed with a powder developer and is, thereby, transformed into a visible image. Further, after the powder-developed image on the photosensitive drum has been transferred onto a sheet, this sheet is separated from the drum, and the powder-developed image on the sheet is fixed.
In this developing step, the photosensitive drum is supplied with a developer composed of carriers and toners or a developer composed of only toners. With this processing, the toners existing in an interior of the developing unit are consumed. When the toners in the interior of the developing unit are totally consumed up, the developing process cannot continue. For this reason, it is required that an internal toner-empty state be detected.
FIGS. 17A and 17B are explanatory diagrams of the prior art. FIG. 18 is a characteristic diagram of a sensor output. As illustrated in FIG. 17A, according to a conventional toner-empty detecting mechanism, a toner sensor 91 is attached to a toner hopper 90 for housing the toners in the developing unit. This toner sensor 91 generates a voltage output corresponding to a toner quantity. Provided further is a comparator 92 for comparing a detected voltage with a threshold voltage. As shown in FIG. 17B, this comparator 92 compares the detected voltage (sensor voltage) with the threshold voltage and, when the detected voltage comes to the threshold voltage or under, outputs a toner-empty signal.
This toner sensor 91 involves the use of, e.g., a magnetic permeability sensor for detecting magnetic toners. In this sensor 91, as illustrated in FIG. 18, when a toner quantity is small, a sensor voltage is set to exhibit a linear characteristic corresponding to the toner quantity. Hence, in the case of the toner quantity being small, the above toner-empty state can be detected.
The output voltage of the toner detecting sensor is, however, on the order of 5 V at most. Accordingly, there is a scatter in terms of the output voltage of the toner detecting sensor, and, therefore, a conventional method based on a fixed threshold voltage is incapable of detecting the toner-empty state with an accuracy. Especially when the toner detecting sensor is mounted on the side of the apparatus with the developing unit being detachably attached, the output voltage becomes still smaller, and hence a greater influence is on the output voltage.